My daughter (8) recently got signed up with a legit agency. There are lots out there but you need to know who they are. If you're on FB there is a great group called Parents of child models and actors UK - PLEASE READ THE FEATURED POST.
The legit agencies don't charge anything to join (sometimes a small admin charge) and you don't need a professional portfolio to sign up- they usually need headshots taken at home under specific conditions and there is usually a self tape to submit - a monologue or something to show their acting